Method of temperature compensation for optoelectronic components, more specifically optoelectronic semiconductors

ABSTRACT

The proposed method of temperature compensation for opto-electronic devices, more specifically opto-electronic semiconductor devices, involves operation of the device under predetermined constant conditions, where a first temperature dependent characteristic value is measured which is then compared with a comparison value determined under identical constant conditions but at a different temperature. A correction function is derived from the relationship between the characteristic value and the comparison value and used to correct the measured value obtained from the semiconductor device so as to compensate for the effect of temperature.

This invention relates to a method of temperature compensation foropto-electronic devices and more specifically for temperaturecompensation of opto-electronic semiconductor devices.

Opto-electronic devices are devices wherein either electrical energy isconverted into light energy or light energy is converted into electricalenergy. In modern technology, opto-electronic semiconductor devices havegained particular importance, namely, on the one hand, particularly thelight-emitting diodes (LEDs) as light-emitting elements and, on theother hand, photo-diodes, photo-transistors, photo-resistors,photo-thyristors and the like, which sense the intensity of lightimpinging onto a measuring surface and output a representativeelectrical signal.

In the following, the method according to the present invention isdescribed by means of an example only with respect to light-emittingdiodes to be referred to as LEDs, and with respect to photo-diodes, butit will be understood that it can also be applied to otheropto-electronic devices and more specifically to opto-electronicsemiconductor devices.

The problem of the application of LEDs and sensor diodes in opticalmeasurement techniques is explained in the following, using colormeasurements as an example: The color impression which a colored surfaceprovides to an observer is based on a predetermined spectraldistribution of the light reflected from said surface which in the eyeof the observer is recognized as color. In this respect, thecolor-sensing function ψ(λ) as seen by the observer is given by:

    ψ(λ)=r(λ)·S(λ)

r(λ) being representative of the remission spectrum of the surface andS(λ) being representative of the spectral distribution of the lightimpinging onto the surface. In other words, the color-sensing functionas seen by the observer is a product of the spectral distribution of thereflection properties of the surface and the spectral distribution ofthe light impinging onto the surface. A changed spectral distribution ofthe light impinging onto the surface will result in a changed colorimpression for the observer.

In technology it is of great importance to correctly detect the color ofsurfaces, namely, on the one hand in order to reproducibly providecolors and on the other hand in order to correctly reproduce color ofsurfaces in printing products, films, photographs and by means ofelectronic devices such as cameras, television screens, computermonitors, and the like.

In conventional color measurements the surface the color of which is tobe detected is irradiated with light having an accurately known spectraldistribution. The reflected light is spectrally analyzed, for exampleusing a spectral photometer, whereby the spectral reflection propertiesof the surface and correspondingly the color impression provided by thesurface can be computed and can be displayed and compared withnormalized color characteristic values.

In order to reduce the apparative efforts for such measurement devices,for some time it has been practiced to irradiate the surface to bemeasured using LEDs, and to measure the light reflected from saidsurface using semiconductor sensors, particularly photo-diodes. Forexample, such a device is disclosed in DE 42 02 822 A1. Therein, aplurality of LEDs arranged on a common substrate as well as a pluralityof sensor diodes are used. The problem with these devices, however, isthe fact that the spectral characteristics and the intensity of both theLEDs and the sensor diodes is temperature dependent such that thetemperature of the LEDs and the sensor diodes has to be detected inorder to enhance the measurement accuracy.

In the device mentioned above this is achieved by arranging atemperature sensor on the common substrate. In a control device of theapparatus, a plurality of spectral characteristics for the LEDs and thesensor diodes is stored, and for every measurement first the temperatureof the substrate is determined and then the corresponding curve for theanalysis of the measurement is selected.

This method firstly comprises the disadvantage that use of a temperaturesensor in the measurement device is relatively expensive. Above all,there is the more important disadvantage that it takes a certain periodof time until a temperature exchange has taken place in the measurementdevice such that all semiconductor devices exhibit the same temperature.Particularly, this involves problems because the LEDs warm up duringoperation. However, until this warming-up of the LEDs has been reliablydetected by the temperature sensor, the measuring procedure has alreadybeen finished.

It is an object of the present invention to provide a method oftemperature compensation for opto-electronic devices and morespecifically for opto-electronic semiconductor devices which enables afast and precise compensation of temperature changes of thesemiconductors.

According to the present invention, this object is achieved by thesubject matter of claim 1.

Preferred embodiments of the invention are listed in the dependentclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the present invention will be described with referenceto the accompanying drawings, wherein:

FIG. 1 illustrates the relation between the forward voltage and theenvironment temperature for the LED, where in the diagram the change ofthe forward voltage is depicted on the ordinate and the change of thetemperature is depicted on the abscissa; and

FIG. 2 illustrates the relation between the measured sensor signal of asensor diode and the forward voltage at the same temperature.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be explained with respect to thecompensation of the temperature dependent drift of light-emitting diodes(LEDs).

According to an alternative of the invention, the LED is operated usinga constant current supply. This means that the current supply of the LEDis provided with a circuit (known in the state of the art) whichgenerates a predetermined constant current. At the same time, theforward voltage of the LED is measured.

The predetermined current shows a dependence on the forward voltagewhich depends on the environment temperature and which is graphicallyillustrated for the range between +10° C. to 40° C. in FIG. 1.

The change of the forward voltage with temperature depends on therespective LED type and generally amounts to 2 to 10 mV per 1° C.

The temperature compensation of the LED is such that every time the LEDis activated with a constant current, at the same time the forwardvoltage is measured. The measuring value of the forward voltage ispreferably stored and the spectral characteristics of the LED used forfurther analysis of the measurement is determined from the measuredvoltage value. Since the LED warms up during operation, this measurementcan reliably take into account the respective temperature, andsimultaneously the respective spectral distribution or intensity of theLED can also be detected and taken into account.

Alternatively, the LED can be operated with a constant voltage sourceand the current can be measured. The corresponding analysis is performedin the same way using the measured current value.

In the following the application of the invention to temperaturecompensation of sensor diodes is described. Also for the sensor diodethe forward voltage depends on the temperature when the sensor diode isoperated as a diode, i.e. in forward direction, using a constant currentsupply. This dependence may be expressed as: ##EQU1## K_(b) is theBoltzmann constant, e is the elementary charge, I_(o) is the essentiallytemperature independent current without external field (in equilibriumwith the diffusion current), I is the externally applied constantcurrent, U_(o) is a material dependent offset voltage, and T is thetemperature of the semiconductor diode.

At constant current, the temperature coefficient of the sensor diodeonly depends on material constants. Thus, equation 2 reduces to:

    U=α·T+C                                     (3)

or

    U.sub.x -U.sub.o =α(T.sub.x -T.sub.o)                (4)

U_(x) represents the forward voltage of the sensor diode at the constantcurrent and at the temperature T_(x), U_(o) represents the forwardvoltage of the diode at constant current and at temperature T_(o)(reference temperature), and a represents the temperature coefficient ofthe sensor diode.

Rearranging this equation results in equation: ##EQU2## This means thatusing this equation by means of measuring the forward voltage of thediode, the temperature of the diode can be determined.

The temperature coefficient α can be determined by measuring the forwardvoltage of the sensor diode at constant current and at differentenvironment temperatures. Since the relation between the change of theforward voltage and the temperature is quite linear, the detection oftwo points and lying a straight line between these two points appears tobe sufficient.

For temperature measurement, i.e. for measuring the forward voltage, thesensor diode is changed from the actual measuring operation which takesplace in the reverse direction of the diode and in which the intensityof the light impinging onto the surface is measured, to the forwardoperation and the sensor diode is supplied with constant current. Theforward voltage measured at this time is a measure of the respectivetemperature and will be taken into account for the analysis of thesensor signal.

The detection of the temperature dependence as explained above can beperformed by changing the environment temperature. In the manufacture ofoptical devices in which such sensor diodes are used, it is quiteexpensive to detect the temperature dependence for each sensor byvarying the environment temperature. Thus, according to the presentinvention, a method is proposed wherein the dependence of the sensorsignal on the temperature may be determined.

In accordance with this method, a sensor is irradiated with a constantlight source, being relatively simple to realize in measurementtechnique.

The diode is operated in measuring condition at the environmenttemperature T_(o) prevailing at the starting point, and the sensorsignal S_(o) is measured which provides the sensitivity of the sensorfor the constant light source at this environment temperature. Then, thesensor diode is changed into forward operation and operated with a smallcurrent I_(o). For this current, the forward voltage U_(o) is measured.

Then, the sensor diode is operated with a remarkably higher current inforward direction for a short period of time, this current beingselected such that it warms up the sensor diode.

After the warming-up period, again the same small current I_(o) is used,and the forward voltage U₁ is measured. Since the sensor diode now has ahigher temperature T₁, the forward voltage is lower than the forwardvoltage U_(o) for the measurement at the starting temperature T_(o).Then the diode is changed to the measurement operation, and now thesensor signal S₁ for the constant light source is recorded.

From this measurement procedure, two pairs of values are obtained,namely, the first measured sensor reading S_(o) at the forward voltageU_(o) and the second measured sensor reading S₁ at the forward voltageU₁. Since the change of the forward voltage with temperature is linear,it is possible to put a straight line through these two points asdepicted in FIG. 2. This straight line gives the relation between theforward voltage and the sensor reading at the same light intensity. Inthis respect, the following equation is approximately valid:

    S.sub.1 =S.sub.o +α(U.sub.1 -U.sub.o)                (6)

For the temperature coefficient α one obtains: ##EQU3## Consequently, inthe measurement according to this embodiment of the inventive method,only the forward voltage has to be determined after recording therespective measurement signal with the constant small current. Thisforward voltage may be used to correct the measured signal. The methodexhibits the major advantage that as correcting quantities directlysensor reading and forward voltage may be used, rather than the value ofthe temperature which the sensor diodes exhibited when changing theforward voltage. Thereby, a very precise connection between the forwardvoltage and the sensor signal may be determined without performingpossibly unprecise and, due to compensating times, also timely expensivetemperature measurements.

In the previously mentioned embodiment, a measurement has been conductedusing a constant current.

Alternatively, for a photo diode operated in reverse direction, also aconstant voltage supply may be used, in which case instead of thetemperature dependent forward voltage, the temperature dependent offsetcurrent is measured in the same manner. In this case, also a temperaturechange may be effected by shortly warming up the diode using a currentflowing in forward direction.

Alternatively, in all method alternatives mentioned above, a smallheating element may be used for warming up the diode or thecorresponding element said heating element being arranged on said sensordiode.

The method also has the advantage that it can be conducted in a verysimple manner regarding the measurement technique, which enables use ofthe method especially in the case when the sensor diodes are alreadymounted in a corresponding apparatus.

I claim:
 1. A method of temperature compensation for opto-electronicdevices, particularly for opto-electronic semiconductor devices,characterized in that said devices are operated under predeterminedconstant conditions and a first characteristic value is measured whichis temperature dependent, and that this characteristic value is comparedwith a comparison value which has been determined under the sameconstant conditions but at a different temperature, and that from therelation between this characteristic value and this comparison value acorrection function is deduced by which the measurement value detectedby the semiconductor device is corrected such that the temperaturedependence is compensated for.
 2. A method of temperature compensationof opto-electronic devices according to claim 1, wherein saidpredetermined constant condition is a constant current which saiddevices are operated with.
 3. A method according to claim 1, whereinsaid predetermined constant condition is a constant voltage which isapplied to said devices.
 4. A method according to claim 1, wherein saidsemiconductor devices are light-emitting diodes (LEDs), that theseconstant conditions are a constant current which said LEDs operatedwith, and wherein this first characteristic value is a forward voltageat which said constant current is measured.
 5. A method according toclaim 2, wherein said correcting function is determined from a linearrelation between said forward voltage and said temperature.
 6. A methodaccording to claim 2, wherein said correction function is determinedfrom a non-linear relation between said forward voltage and saidtemperature.
 7. A method according to claim 1, wherein saidsemiconductor devices are light-emitting diodes (LEDs), and whereinthese constant conditions are a constant voltage which said LEDs areoperated with, and that said characteristic value is the offset currentwhich is measured at this constant voltage.
 8. A method according toclaim 1, wherein said opto-electronic devices are semiconductor sensordiodes, and wherein said constant conditions is the current which saidsensor diodes are operated with in forward direction, and wherein saidcharacteristic value is the forward voltage measured at said constantcurrent.
 9. A method according to claim 8, wherein for detecting thechange of its characteristics with temperature, the sensor diodes,respectively is changed from measurement operation taking place inreverse direction of the diodes, respectively into temperature detectionoperation in forward direction of the diodes, respectively.
 10. A methodaccording to claim 8 or 9, wherein said correction value is alight-intensity correction value which is detected by measuring thelight intensity of a constant light source, once at a low temperatureand once at a higher temperature.
 11. A method according to claim 10,wherein said higher temperature of said sensor diodes, respectively isprovided by operating said sensor diodes, respectively with a highercurrent in forward direction for a short period of time.
 12. A methodaccording to claim 11, wherein said higher temperature is provided byheating said diodes with a heating element, a resistor or the like.